Low volume - bass breaking through
 

I am having an issue with bass "breaking through" and causing my subs to output a faint sound only at low volume levels. It is almost as if a tiny amount of bass was barely able to trigger output from the amplifier. Is sounds like a very faint farting sound that is output with the beat of the song. This happens with the engine on or off. I have

Also, if I place an ear close to the the sub, I hear what I believe is engine noise. I replaced the standard RCA running between my Alpine Theatre amp and MMATS sub amp with a homemade Cat5e cable and this reduced the noise quite a bit. There is still some noise present - again, only if you place an ear within inches of the sub. Both amps are grounded to the same point and it was properly done. I have not done the "big 3" so I am not sure if the stock ground system is decent.

Any advice?

Thanks

JJ